Materials used in the manufacture of electronics enclosures, e.g. frame equipment or other equipment for telecommunications equipment, and steel enclosures for housing printed circuit boards and circuit packs, must be protected against corrosion, particularly for outdoor exposure. Conventionally, the walls of the enclosures have been coated with zinc plating, which in turn has been protected with a chromate conversion coating of varying thickness. The chromate protected zinc coating provides sufficient rust and corrosion protection to the metal of the enclosures for many applications. However, when grounding of such enclosures is required, e.g. to provide Faraday enclosures for electromagnetic shielding of components, it is found that good electrical contacts cannot be made reliably between components having conventional chromate coatings.
Consequently, various approaches have been taken to obtain electrical contact through the coating, i.e. by piercing the coating. For example, special beryllium copper gaskets having sharp spurs may be used to make contact between coated components to providing grounding. However, since the coating is destroyed at the contact points, these areas are left open to corrosion. One solution is to provide some form of local protection, e.g. in the form of tin lead solder at the connection points.
Nevertheless, this structure is not ideal, and electronics enclosures of this type have found to be lacking in meeting current requirements for shielding and improved control electromagnetic interference (EMI). Shielding may be required to protect components in the enclosure from EMI emanating outside the enclosure or for EMI from equipment housed inside the enclosure.
Furthermore, while chromate coatings are satisfactory in providing corrosion resistance that meets industry standard tests, e.g. for exposure to 100 hrs and 300 hrs of salt fog, chromate is known to be toxic to man and the environment, and other less environmentally harmful alternatives are now being sought.
Various conversion coatings have been tested under the auspices of the National Consortium of Manufacturing Sciences and found to be either too complicated, toxic or ineffective. It is also known that various other active metal oxides have been investigated as alternatives to chromate. For example, vanadium, manganese, tungsten oxides have been tested and generally found to be more toxic or polluting than the conventional chromate coatings.
Industry interest in alternative coatings to replace chromate and other existing coatings such as nickel coatings, is increasing in view of environmental initiatives to reduce use of various contaminants, including chromium and nickel, amongst others, such as heavy metals, which are subject to special waste disposal requirements or environmental regulations.
U.S. Pat. No. 5,607,521 entitled `Method for post-treatment of an article with a metallic surface as well as a treatment solution to be used in the method` to Danish Instituttet for Produckudvikling (IPU), describes a molybdenum phosphate coating with improved corrosion resistance to provide an alternative to conventional chromate coatings. It is believed that such a molybdenum phosphate (`MolyPhos") coating has a lower toxicity than conventional chromate coatings.
Copending U.S. patent application Ser. No. 08/995,410 filed Dec. 19, 1997, now U.S. Pat. No. 5,981,871, entitled `Electronics enclosures` to the present inventors Trumble et al., which is incorporated herein by reference, describes use of such a MolyPhos conversion coating over a conventional zinc plating, to overcome some of the above mentioned problems in providing electrical connections between coated components of an enclosure, and a method of providing such an enclosure using a MolyPhos coating. Since the MolyPhos coatings also provide good corrosion resistance over zinc, these coatings are desirable alternatives to conventional chromate coatings applications such as electronic enclosures with EMI shielding for telecommunications equipment.
However, while these MolyPhos coatings overcome some disadvantages with chromate coatings, tests results for corrosion resistance of MolyPhos, e.g. the resistance to some industry standard tests such as extended salt fog exposure, suggests that some enhancements or improvements to the existing MolyPhos process are desirable to improve corrosion resistance for more hostile environments, and to meet the more stringent requirements for applications for the automotive and aviation industry.